Some microphones used in meetings are boundary microphones that are placed on desktops for sound collection. Some boundary microphones have a function that allow speakers to operate the switches of the microphones. Such an operation of a switch generates sound and vibration in the boundary microphone. The sound and vibration are transmitted to a microphone unit inside the boundary microphone. As a result, the microphone unit generates noise. Various boundary microphones including membrane switches for preventing such noise have been proposed (for example, refer to Japanese Patent Publication No. 5534822).
FIG. 4 is a cross-sectional side view illustrating a conventional boundary microphone.
The boundary microphone includes a base 2a, a cover 3a, a button 4a, a circuit board 5a, a microphone unit 6a, a cord bush 7a, a microphone cable 8a, a threaded screw T1, and a threaded screw T2. The boundary microphone is placed on a mounting surface G, such as a desktop.
The base 2a is composed of metal. The base 2a has a flat shape with an exposed upper face (the upper side in FIG. 4). The base 2a has a depression and a cavity 9a. The depression is disposed in the upper face of the base 2a. The cavity 9a is provided between the depression and the exposed portion of the upper face of the base 2a (hereinafter referred to as “exposed portion”).
The cover 3a covers the exposed portion of the upper face of the base 2a. The cover 3a is composed of metal. The cover 3a has multiple acoustic-wave entering holes. The cover 3a is fixed to the base 2a with the threaded screw T1.
The button 4a is supported in the depression in the base 2a with a support composed of an elastic material, such as rubber. When the button 4a is pressed by a finger F of an operator, then the button 4a depresses into the depression in the base 2a. When the button 4a is released from the pressing by the finger F of the operator, then the button 4a returns to the position prior to the pressing.
The base 2a and the cover 3a define a space inside the boundary microphone. The circuit board 5a and the microphone unit 6a are accommodated in this space.
The circuit board 5a is fixed to the base 2a with the threaded screw T2. The circuit board 5a is electrically connected to one end of the microphone cable 8a. The other end of the microphone cable 8a is led out from the base 2a through the cord bush 7a. 
The circuit board 5a is provided with various electric circuits that process electrical signals output from the microphone unit 6a to generate audio signals. The audio signals generated by the circuit board 5a are output to external processors for audio signals via the microphone cable 8a. 
The microphone unit 6a is, for example, a condenser microphone unit. The microphone unit 6a includes a diaphragm and a fixed electrode, which constitute a condenser. The diaphragm receives acoustic waves passing through the acoustic-wave entering hole in the cover 3a and vibrates. The microphone unit 6a converts the variation in the capacitance between the diaphragm and the fixed electrode, which constitute a condenser, to electrical signals and outputs these electrical signals. The microphone unit 6a is electrically connected to the circuit board 5a. 
The cord bush 7a is disposed on the base 2a. The position of the cord bush 7a on the base 2a (at the right in FIG. 4) is opposite to the position of the button 4a. The microphone cable 8a passes through the cord bush 7a. 
The button 4a is, for example, a part of a pressure-sensitive switch. The pressure-sensitive switch includes a membrane and a circuit board. The circuit board includes a flexible printed board L, which is indicated by the dashed line in FIG. 4. The membrane and the circuit board of the pressure-sensitive switch are attached to the base 2a such that the patterned portion of the circuit board faces the membrane. The pressure-sensitive switch turns on due to contact of the pattern of the circuit board and the membrane.
The flexible printed board L passes through the cavity 9a. The flexible printed board L is electrically connected to the circuit board 5a. As a result, the pressing operation of the button 4a is transmitted to the circuit board 5a via the flexible printed board L.